Altitude control system

ABSTRACT

1. AN ECHO-RANGING SYSTEM COMPRISING IN COMBINATION, A TRANSDUCER, A BLOCKING OSCILLATOR COUPLED TO SAID TRANSDUCER, A TRANSMIT-RECEIVE CIRCUIT EFFECTIVELY CONNECTED TO SAID BLOCKING OSCILLATOR AND SAID TRANSDUCER, TUNED AMPLIFIER MEANS COUPLED TO THE OUTPUT OF SAID TRANSMIT-RECEIVE CIRCUIT, A SHUNT DETECTOR EFFECTIVELY CONNECTED TO THE OUTPUT OF SAID TUNED AMPLIFIER MEANS, A CLIPPER CIRCUIT COUPLED TO THE OUTPUT OF SAID SHUNT DETECTOR, A DIODE DETECTOR CONNECTED TO THE OUTPUT OF SAID CLIPPER CIRCUIT, A ONE-SHOT MULTIVIBRATOR HAVING A PAIR OF INPUTS AND AN OUTPUT WITH ONE OF THE INPUTS THEREOF COUPLED TO THE OUTPUT OF THE AFORESAID BLOCKING OSCILLATOR AND WITH THE OTHER INPUT THEREOF COUPLED TO THE OUTPUT OF SAID DIODE DETECTOR, AND A ZENER LIMITER EFFECTIVELY CONNECTED TO THE OUTPUT OF THE AFORESAID ONE-SHOT MULTIVIBRATOR.

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ALTITUDE CONTROL SYSTEM United States Patent O 3,559,157 ALTITUDECONTROL SYSTEM Frederick B. Cupp, Willoughby, and Carl W. Palley,Mentor, Ohio, assignors, by mesne assignments, to the United States of'America as represented by the Secretary of the Navy Filed May 28, 1964,Ser. No. 371,155 Int. Cl. G01s 9/68 U.S. Cl. 340-3 13 Claims The presentinvention relates in general to altitude control systems and inparticular is an improved method and means for ascertaining the depth ofa submarine vehicle and producing an error signal proportional to thedeviation from a predetermined control altitude level. Furthermore, andeven more specifically, it is a transistorized, compact sonarecho-ranging circuit that produces a negative, substantially square wavetype of signal, having a time duration that is a function of thealtitude of the transducer above the seat oor at any given instant.

In the past, altitude control systems and comparable echo-rangingsystems have been employed successfully for many practical purposes.However, for the most part, such systems have been quite large and heavywhich makes them somewhat burdensome to handle and the powerrequirements for the operation thereof were considerable and, likewise,were somewhat burdensome. Moreover, in many instances, the reliabilityof similar prior art devices left a great deal to be desired.

The instant invention overcomes to a considerable eX- tent thedisadvantages of the prior art devices because it is compact, light inweight, susceptible to being manufactured as a printed circuit board,and requires only a small power supply.

It is, therefore, an object of this invention to provide an improvedaltitude control system.

Another object of this invention is to provide an improved contourfollower system.

Another object of this invention is to provide an improved method andmeans of sensing and controlling the depth at which a submarine vehicleruns with respect to the ocean bottom.

Another object of this invention is to provide an improved method andmeans for automatically maintaining a towed depth charge within aspecified stratum above the bottom of the sea.

Still another object of this invention is to provide an improvedecho-ranging system.

A further object of this invention is to provide a transistorizedtransmit-receive circuit for an echo-search system.

A further object of this invention is to provide an improvedtransistorized echo-ranging system that is small in size, light inweight, and reliable in operation.

Another object of this invention is to provide an improvedtransistorized echo-ranging system that has increased accuracy as aresult of temperature compensation being incorporated therein.

Still another object of this invention is to provide a transistorizedtransmit-receive circuit that may be readily mounted as a printedcircuit board.

Another object of this invention is to provide an echoranging systemthat has only relatively small power requirements.

Other objects and many of the attendant advantages will be readilyappreciated as the subject invention be- 3,559,157 Patented Jan. 26,1971 ICC ' vention.

Referring now to FIG. l, there is shown an altitude control systemcomprising a tuned free-running, blocking oscillator 11 with the outputthereof connected through a toroidal transformer 12 to the input of anelectroacoustical transducer 13. Transducer 13 is the reversible typethat converts electrical energy into proportional acoustical energy andvice versa. The output thereof is applied to the aforesaid transformer12 during receiving operations, as will be further explainedsubsequently.

Toroidal transformer 12, thus, has one of its outputs coupled to atransmit-receive (TR) circuit 14 which is adapted for passing allreceived echo signals but stops practically all originally transmittedsearch signals so that they will not be processed as received signals orotherwise cause signal distortion to exist. The output of TR circuit 14is coupled through a tuned amplifier 15, a tuned transformer 16, anothertuned amplifier 17, another tuned transformer 18, and another amplifier19 to an emitter follower 21.

The output of emitter follower 21 is supplied to an amplifier 22, theoutput of which is coupled to a shunt detector 23 which, of course, isso designed as to shunt to ground that component of the echo signalhaving the same frequency as the output signal from blocking oscillator11. The output from shunt detector 23 is then clipped by aclipper-limiter 24 and again detected by a diode 25 so as to produce adirect current envelope output signal therefrom. This DC envelope outputsignal is supplied to one of the inputs of a one-shot multivibrator 26to timely turn it off after it was originally triggered and turned on bya transmit pulse supplied thereto from the aforesaid blocking oscillator11, as a result of the original search signal transmission,

The output of multivibrator 26 is then fed through a circuit isolationemitter follower 27 before being limited by Zener diode limiter 28,after which it is coupled through a relay 29 to the input of autilization apparatus controller 31. While, in this particular case, theinput signal to controller 31 is supplied from relay 29, it could beobtained for many suitable purposes from the output of Zener limiter 28,since so doing would obviously be well within the purview of one skilledin the art having the benefit of the teachings herein presented.

The input to controller 31 is actually supplied to a squarewave detectorand control amplifier 32, the output of which is connected to up-downrelays 33 which, in turn, operate the motor of a winch 34. A cable 35from winch 34 is connected to the towed submarine vehicle or otherutilization apparatus whose altitude above the sea fioor is beingcontrolled. Of course, pulling in said cable raises the vehicle andpaying out said cable causes it to be lowered.

In order to improve the accuracy of altitude control, a plurality ofbias voltage regulation thermistors 36, 37, and 38 are respectivelyincorporated in the circuits of tuned amplifier 15, tuned transformer16, and one-shot multivibrator 26. Thus, temperature compensationtherefor is effected which, in turn, effects improved circuit stabilityand overall accuracy, even during ambient temperature liuctuations.

Referring now to the detailed circuit diagram of FIGS. 2A and 2B, theecho-ranging portion of the subject invention is disclosed as includinga B- voltage 40 and a ground 41. Between said B- voltage and ground, apower supply filter 42, containing a pair of voltage stabilizationcapacitors 43 and 44, is connected, and to the negative side of saidstabilization capacitors is connected the collector of a 2N174A PNPtransistor 45 of blocking oscillator 11. The base thereof is connectedthrough a resistor 46 to the side of stabilization capacitors 43 and 44.Likewise, the base of transistor 45 is coupled through a capacitor 47 toone terminal of a primary winding 48 of the aforementioned toroidaltransformer 12, with the other end terminal of said transformer coupledto ground. The emitter of transistor 45 is coupled to a center tap ofsaid primary winding 48 of transformer 12. A secondary winding 49thereof has one terminal connected to the aforesaid transducer 13 andthe other terminal thereof is coupled to the aforementionedtransmit-receive circuit 14, consisting of a pair of reversed polaritydiodes 51 and 52 that are connected in parallel with each other and inseries with a capacitor 53, the positive side of which is coupled toground. Another voltage stabilization'capacitor 54 iS included in tunedamplifier circuit 15 along with a 2N33l PNP transistor 55. The base oftransistor 55 is coupled through a resistor 56 to a slightly reduced B-voltage Reduction in said B- voltage is, of course, effected by aresistor 57 inserted in the B- voltage line. Since for all practicalpurposes said reduced B- voltage still constitutes a B- voltage, it willbe referred to in that manner in the remaining discussion of the subjectcircuit to follow.

The base of the aforesaid transistor 55 of tuned amplilier 15 is alsoconnected through a thermistor 36 to ground, and a capacitor 58 isparallel coupled therewith. The emitter of transistor 55 is coupledthrough a resistor 59 to ground and is also connected to the negativevoltage terminals of the aforesaid reversed polarity diodes 51 and 52,since these terminals effectively constitute the output of the aforesaidtransmit-receive circuit 14. The collector of transistor 55 is connectedto a predetermined intermediate turn of a primary winding 61 of tunedtransformer 16. Couped in parallel with said winding 61 is a capacitor62, and between one terminal thereof and the aforesaid B- voltage isconnected resistor 63.

The output of transformer 16 is taken across a secondary winding 64thereof, one terminal of which is connected through parallel connectedthermistor 37 and capacitor 65 to ground, and through a resistor 67 tothe B- voltage, with the other terminal thereof connected t the base ofa 2N331 PNP transistor 66 incorporated in the aforementioned tunedamplifier 17. The emitter of transistor 66 is connected through aparallel connected resistor 68 and capacitor 69 to ground, and thecollector thereof is connected to a predetermined intermediate turn of aprimary winding 70 of the aforementioned tuned transformer 18. The endterminals of primary winding 70 has coupled in parallel therewith acapacitor 71 and a diode 72, and one terminal thereof is connectedthrough a resistor 73 to B-. A secondary winding 74 of transformer 18has one of the terminals thereof connected through a resistor 75 to B-and through a resistor 76 and a capacitor 77 parallel connectedtherewith to ground.

The other terminal of secondary winding 74 of transformer 18 is coupledto the base of a 2N331 PNP transistor 78 which is incorporated in theaforementioned amplifier circuit 19. The emitter thereof is connectedthrough parallel connected resistor 79 and capacitor 81 to ground, andthe collector thereof is connected through a resistor 82 to B-.

A coupling capacitor 83 is connected between the collector output oftransistor 78 and the base input of another 2N331 PNP transistor 84 ofthe aforementioned emitter follower circuit 21. The base of transistor84 is also coupled through a resistor 85 to B- and through anotherresistor 86 to ground. The collector of transistor 84 is direct coupledto the B- voltage, and the emitter thereof is coupled through a resistor87 to ground. Inasmuch as transistor 84 is incorporated in an emitterfollower circuit, the output is taken from the emitter thereof andcoupled through a coupling capacitor 88 to the base of a 2N33l PNPtransistor 89.1ocated in the aforementioned amplifier circuit 22, biasedby voltage divider network consisting of resistors 80'and 90. Theemitter thereof is coupled through a resistor 91 and capacitor 92connected in parallel therewith to ground, and the collector thereof isconnected through a resistor 93 and a capacitor 94 connected in paralleltherewith, said resistor 93 and capacitor'94 constituting theaforementioned shunt detector 23.

The output of amplifier 22 is taken from the collector of transistor 89and is coupled through a coupling capacitor 95 to the input of clippercircuit 24 consisting of parallel connected resistor 96 and diode 97,one terminal of which is coupled to ground. The output of capacitor 95is likewise coupled through the aforementioned diode 25 and a resistor98 to ground, and the junction of said diode 25 and resistor 98 isconnected through another capacitor 99 to the input of theaforementioned one-shot multivibrator 26.

Actually, the output of diode detector 25 is effectively supplied to thebase input of a 2N33l PNP transistor 101. The base thereof is alsoconnected through the aforementioned thermistor 38 and a resistor 102connected in parallel therewith to ground. The emitter of transistor 101is coupled through a resistor 103 to ground and to the emitter ofanother 2N331 PNP transistor 104. The collector of transistor 101 isconnected through a resistor 105 to the aforesaid reduced B- voltage.Furthermore, said collector is coupled through a series connectedresistor 106 and capacitor 107 to the emitter of PNP transistor 45 ofblocking oscillator circuit 11 mentioned above. Connected between thejunction of resistor 106 and capacitor 107 and ground is anothercapacitor 108.

The collector of transistor 101 is coupled through a pair of couplingcapacitors 109 and 111 to the base of transistor 104 and through aresistor 112 to B-. The collector of transistor 104 is coupled throughanother resistor 113 to B- and through another resistor 114 to theoutput of the aforementioned capacitor 99 immediately following diodeddetector circuit 25. Of course, said transistors 101 and 104 and theirassociated circuitry constitutes one-shot multivibrator 26.

The output of multivibrator 26 is taken from the collector of transistor104 and is fed through a resistor 115 to the base of a 2N33l PNPtransistor 116 located in emitter follower circuit 27. The collector oftransistor 116 is coupled directly to the reduced B- voltage, and theemitter thereof is coupled through a resistor 117 t0 ground. Since theemitter of transistor 116 constitutes the output of emitter follower 27it is coupled through a coupling capacitor 118 to Zener diode 28 whichlikewise has its other terminal connected to ground. As may readily beseen, diode 28 constitutes the aforementioned Zener limiter which limitsthe output voltage to an optimum predetermined value.

The output of Zener limiter 28 is taken from the junction thereof andcapacitor 118 and is supplied through a; resistor 119 to stationaryswitch contact 121 of the aforementioned relay 29. Actually, if sodesired, the output from either Zener limiter 28 or resistor 119 may beconsidered the output of the echo-ranging portion of the subjectinvention. However, in this particular case, said output is preferablysupplied through relay 29 in order to facilitate the coupling thereof tothe particular utilization apparatus controller depicted in FIG. 1 andfor other functional purposes not included herewith. Hence, said relaymay or may not be included within the subject invention, since so doingin either case would obviously be well within the purview of one skilledin the art having the benefit of the teachings herein presented, inorder to provide optimum use thereof under any given operablecircumstances.

In order to be consistent with the device of FIG. 1 and to providecontinuity between the echo-ranging portion of this invention andutilization appartus controller 3 1 herein employed, relay 29 will bedescribed in brief. As previously mentioned, the output of Zener limiter28 is effectively applied to stationary switch contact 121 of relay 29.Relay 29, of course, has a pair of single throw-double pole switches 122and 123 which are actuated by a solenoid 124. Coupled in parallel withsolenoid 124 is a diode 125 and one terminal thereof is coupled to areversely polarized diode 126fto lground in order to insure that theappropriate direct current is supplied to solenoid 124, inasmuch as itis preferably a direct current solenoid. The circuit of solenoid 124 iscompleted by connecting one terminal thereof to a fixed switch contact127 of switch portion 123 and the movable arm contact 128 of switchportion 122. In this particular case, another stationary switch contact|129 of relay switch 122 is left blank, but, if so desired, may beutilized for supplying electrical current to any other desiredutilization apparatus not shown herein. Another stationary contact 131of relay 29 is coupled to the aforementioned B-, and a movable arm 132thereof (ganged with movable arm 128) is connected to the negative poleof a 12 volt direct current battery 133. The positive pole thereof iscoupled through a fuse 134 to the negative pole of another 12 voltdirect current battery 135, the positive pole of which is coupledthrough a pressure switch 136 to ground. Thus, batteries 133 and 135effectively supply the aforementioned B- voltage to the entireecho-ranging circuit. As may readily be seen, the output of relay 29 iseffectively taken from the movable arm 128 of switch 122 and this occursas a result of movable arm 128 and stationary contact 121 normally beingin an initially closed position prior to the energization of solenoid124 by the application of a voltage thereto from some other source (notshown) that is greater than the maximum negative square wave outputvoltage from Zener limiter 28. Thus, in this case, it may be seen thatfwhen relay 29 is energized, the utilization apparatus controller isdisconnected from the output of the echo-ranging circuit.

Briefly, the operation of the subject invention is discussed inconjunction with FIGS. 1, 2A, and 2B as follows:

Freerunning blocking oscillator 11 is preferably designed to produce anoutput signal having a frequency of the order of sixty-five kilocyclesper second. This signal is simultaneously applied through transformer 12to transducer 13 and to one-shot multivibrator 26 to trigger it andthereby produce the front edge of a negative square wave output signal.The negative portion of said square Wave output signal continues induration and, consequently, in length until such time as multivibrator26 is turned off by the received echo signal, unless such durationexceeds twelve milliseconds. Multivibrator 26 is, accordingly, designedto produce a negative going square Wave having a time constant of twelvemilliseconds b ecause this amount of time corresponds to atransmitreceive distance of thirty feet in water, the maximum rangedesired in this particular embodiment.

During the transmission period, transducer 13 broadcasts acousticalenergy into the sea water and, in this case, it is preferably directedtoward the sea fioor so as to effectively determine the distance theretoat any given instant. To substantially eliminate any of the transmittedsignal from being passed on to the receiver portion of this invention,transmit-receive circuit 14, comprising a pair of parallel reversedpolarity diodes (see iFIG. 2A), bypasses to ground all components of theaforesaid slxtyfive kilocycle per second signal having an amplltude orstrength exceeding a predetermined design value. For most practicalpurposes, this limiting or signal-stopping function prevents signaldistortion sufficiently to provide an adequate signal-to-noise ratio;however, subsequent processing and shunt detector 23 improve thesituation by causing the sixty-five kilocycle signal, if any, that gotthrough transmit-receive circuit 14 to be filtered out and shunted toground, thereby preventing an adverse effect on the operation ofone-shot multivibrator' 26 during the receive period thereof.

The echo-signal from the sea fioor (or other target, as the case may be)occurs sometime between the zero time of transmission and expiration oftwelve milliseconds if the altitude or depth being measured is less thanthirty feet. As a result of its being originally broadcast within amulti-molecule subaqueous medium, reflected from a multi-particle seaoor, reflected from a possible target, and perhaps modified due to thepresence of Doppler, said echo signal is received as a multi-frequencyintelligence signal, which ordinarily includes as a component thereofthe originally broadcast seventy-five kilocycle per frequency signal.This echo is sensed by transducer 13, passed through transmit-receivecircuit 14, amplified, filtered, and shaped in tuned amplifier 17, tunedtransformer 18, and amplifier 19, respectively. Then it is passedthrough circuit isolation emitter follower 21 and further amplified byamplifier 22 before being applied to shunt detector 23. 4In shuntdetector 23, any sixty-five kilocycle per second component that maystill exist in the return signal is effectively bypassed to ground, andthe remaining intelligence signal is passed on to be clipped byclipper-limiter 24. Because removal of the broadcast frequency isprimarily important during the transmit situation in order to prevent itfrom triggering multivibrator 26 and turning it off rather than on atthat time, shunt detector 23 was incorporated in the subject system;however, during the receive situation whether or not the broadcastfrequency component is completely removed from the intelligence echosignal being processed at that time is not absolutely critical from anoperational standpoint, since, after processing, the intelligencefrequencies concomitant therewith actually act as multivibratoroperators anyway. Of course, it should be understood that removal of thebroadcast signal from the received echo signal during the processingthereof tends to optimize the operation of the subject invention from asignal-to-noise enhancement standpoint. After clipping, the envelopethereof is extracted by diode detector 25 to produce an appropriatesignal, the trailing edge of which when applied to multivibrator 26turns it off provided, of course, as previously mentioned, it occursprior to the expiration of a twelve millisecond time period.

Accordingly, it may readily be seen that the duration of the negativesquare wave output of multivibrator 26 is a function of the altitude ofthe transducer above the sea floor. This signal is then furtherprocessed by circuit isolation emitter follower 27 and Zener limiter 28,the latter of which limits the output of the entire transmit-receivecircuit to a useful maximum of negative twelve volts.

This negative altitude information output signal is then fed throughrelay 29 to the input of utilization apparatus controller. Although inthis case, said controller included squarewave detector and controlamplifiers 32, up-down relays 33, and winch 34, any suitableconventional controller which is responsive to and controls inaccordance with the length of a negative square wave output signal maybe employed as necessary to fulfill the control requirements of anygiven operational circumstances or applications. In this particularembodiment, however, a submarine vehicle is intended to be raised orlowered to maintain a substantially constant altitude from the seafloor, as it is being towed by cable 35 from some tractor device, suchas a ship or the like.

In order to insure that reflections and/ or reverberations from theinterface of the air and water at the surface of the sea do not have anydeleterious infiuences on the echoranging process of this invention,pressure switch 136 is installed therein in such manner that 24 volts isproperly applied thereto only when the controlled submarine vehicle isat least some predetermined depth below said surface At that time thetransmit-receive portion of the subject invention is energized forecho-ranging and altitude control operations. It should be understood,however, that in the event this feature is not wanted, it would only benecessary to apply 24 direct current volt B- voltage in the appropriatemanner to energize the various and sundry stages as shown in FIG. 2B byeliminating or shorting pressure switch 136.

It is, likewise, to be understood that although relay 29 has `beenincorporated as an intermediate component between the output of theecho-ranging portion of this invention and the input to the utilizationapparatus controller, it may be omitted if desired for many practicalpurposes, inasmuch as the selection thereof is merely a matter of designchoice of the artisan and is contingent upon whether or not otherassociated operations need to be performed. In such instance, the outputwould obviously be taken from the output of Zener limiter 28. On theother hand, if relay 29 is included, the output therefrom may be takenfrom any appropriate switch element thereof as required to properlyeffect performance of said other associated operations.

In this case, the utilization apparatus controller is employed to raiseor lower the towed submarine vehicle. This occurs because the squarewavedetector and control amplifier functions in accordance with the durationof the negative squarewave to cause up-down relays 33 to, in turn, causeone or more winches 34 to pay out or roll in tow cable 3S, and therebylower or raise the submarine vehicle.

`Of course, it would again be obvious to the artisan to make controller31 regulate any other desired appropriate utilization apparatus.

Obviously, many modifications and other embodiments of the subjectinvention will readily come to the mind of one skilled in the art havingthe benefit of the teachings presented in the foregoing description inaccompaniment with the associated drawing. Therefore, it is to beunderstood that the invention is not to be limited thereto and that saidmodification and embodiments are intended to be included within thescope of the appended claims.

What is claimed is:

1. An echo-ranging system comprising in combination,

a transducer,

a blocking oscillator coupled to said transducer,

a transmit-receive circuit effectively connected to said blockingoscillator and said transducer,

tuned amplifier means coupled to the output of said transmit-receivecircuit,

a shunt detector effectively connected to the output of said tunedamplifier means,

a clipper circuit coupled to the output of said shunt detector,

a diode detector connected to the output of said clipper circuit,

a one-shot multivibrator having a pair of inputs and an output with oneof the inputs thereof coupled to the output of the aforesaid blockingoscillator and with the other input thereof coupled to the output ofsaid diode detector, and

a Zener limiter effectively connected to the output of the aforesaidone-shot multivibrator.

2. The device of claim 1 wherein said blocking oscillator includes a PNPtransistor.

3. The device of claim 1 wherein said transmit-receive circuitcomprises,

a first diode,

a second diode connected in parallel with said first diode in a reversedpolarity relationship, and

a capacitor connected in series with said parallel connected first andsecond diodes.

4. The device of claim 1 wherein said tuned amplifier means includes atleast one PNP transistor.

5. The device of claim 1 wherein said shunt detector comprises,

a resistor, and

a capacitor connected in parallel therewith.

6. The device of claim 1 wherein said clipper circuit comprises,

a diode, and

a resistor connected in parallel with said diode.

7. The device of claim 1 wherein said one-shot multivibrator includes apair of set and reset PNP transistors.

8. The device of claim 1 wherein said Zener limiter is a Zener diode.

9. An altitude control system comprising in combination,

means for timely generating a predetermined frequency signal,

means for receiving signals within a predetermined frequency range,

means connected to said receiving means for substantially blockingthereto the aforementioned predetermined frequency signal as it istimely being generated by said generating means,

means coupled to said receiving means for bypassing any of saidpredetermined frequency signals not blocked by said blocking means,

means coupled to said receiving means for shaping the signal therefrom,

means connected to outputs of said predetermined frequency signalgenerating means and said signal shaping means for producing a signalhaving a duration that is proportional to the time duration between thegeneration of said predetermined frequency signal and the reception of amodified version thereof by said receiving means, and

a transducer connected to the output of said predetermined frequencysignal generating means and the input of said receiving means.

10. The invention according to claim 9 further characterized by meansconnected to said receiving means for compensating for changes in theambient temperature thereof.

11. The invention according to claim 9 further characterized by meansconnected to the output of said proportional duration signal producingmeans for isolating the circuit thereof and limiting the amplitude ofthe proportional duration signal therefrom.

12. The invention according to claim 10 further characterized by acontroller means connected to the output of said circuit isolation andlimiting means.

13. In an altitude control system,

means for timely generating a predetermined frequency signal,

means for receiving signals within a predetermined frequency range,

means connected to said receiving means for substantially blockingthereto the aforementioned predetermined frequency as it is timely beinggenerated by said generating means,

means coupled to said receiving means for bypassing any of saidpredetermined frequency signals not blocked by said blocking means,

means coupled to said receiving means for shaping the signal therefrom,

means connected to outputs of said predetermined frequency signalgenerating means and said signal shaping means for producing a signalhaving a duration that is proportional to the time duration between thegeneration of said predetermined frequency signal and the reception of amodified version thereof by said receiving means,

a transducer connected to the output of said predetermined frequencysignal generating means and the input of said receiving means,

10 a first thermistor connected to said receiving means, 3,061,81210/1962 Rachwalski 340-3 and 3,117,241 1/1964 Paynter et al. 307-885 asecond thermistor connected to the aforesaid propor- 3,153,220 10/1964Hagemann 340--3 ional duration signal producing means. OTHER REFERENCESReferences Cited 5 Mitchell, Transistorized Fish Finder, ElectronicsWorld, vol. 62. UNITED STATES PATENTS u Krauth A- Pllfl'lary EXamlner2,591,856 4/1952 Oliver 343-13 10 Us' CL X'R` 2,724,816 11/1955 Goodell340-3 343- 13

